valgrind - valgrind中可能丢失的含义是什么

Question

我有很多可能从 valgrind 丢失的条目。这意味着什么 ？
因为我正在使用 sqlite 并且它经过了很好的测试。我不认为这些是正确的条目。我做错了什么？

 16 bytes in 1 blocks are possibly lost in loss record 30 of 844
    ==23027==    at 0x4A05E1C: malloc (vg_replace_malloc.c:195)
    ==23027==    by 0x6525BE: sqlite3MemMalloc (in app_mem.out)
    ==23027==    by 0x63C579: mallocWithAlarm (in app_mem.out)
    ==23027==    by 0x63C904: sqlite3DbMallocRaw (in app_mem.out)
    ==23027==    by 0x6886D6: codeOneLoopStart (in app_mem.out)
    ==23027==    by 0x68A9C8: sqlite3WhereBegin (in app_mem.out)
    ==23027==    by 0x68CC9E: sqlite3Select (in app_mem.out)
    ==23027==    by 0x6A8644: yy_reduce (in app_mem.out)
    ==23027==    by 0x6AAEAC: sqlite3Parser (in app_mem.out)
    ==23027==    by 0x6AB357: sqlite3RunParser (in app_mem.out)
    ==23027==    by 0x6ADF84: sqlite3Prepare (in app_mem.out)
    ==23027==    by 0x6AE82B: sqlite3LockAndPrepare (in app_mem.out)

Answer 1

Valgrind 源代码 3.6.1 版中包含的常见问题解答确实详细说明了一点:

"possibly lost" means your program is leaking memory, unless you're doing unusual things with pointers that could cause them to point into the middle of an allocated block; see the user manual for some possible causes. Use --show-possibly-lost=no if you don't want to see these reports.

(5.2. 杂项，Valgrind 常见问题解答)

Valgrind 用户手册讨论了它如何跟踪使用 malloc/new 分配的所有堆 block ，并描述了两种跟踪内存的方法:

通过维护指向内存块开始的“开始指针”

通过维护指向 block 中间某个位置的“内部指针”

可能出现内部指针的三种情况:

该指针可能最初是一个起始指针，并且已被程序故意(或非故意)移动。

它可能是内存中的随机垃圾值，完全不相关，只是巧合。

它可能是指向使用 new[] 分配的 C++ 对象(具有析构函数)数组的指针。

可能的场景:

     Pointer chain            AAA Category    BBB Category
     -------------            ------------    ------------
(5)  RRR ------?-----> BBB                    (y)DR, (n)DL
(6)  RRR ---> AAA -?-> BBB    DR              (y)IR, (n)DL
(7)  RRR -?-> AAA ---> BBB    (y)DR, (n)DL    (y)IR, (n)IL
(8)  RRR -?-> AAA -?-> BBB    (y)DR, (n)DL    (y,y)IR, (n,y)IL, (_,n)DL

Pointer chain legend:
- RRR: a root set node or DR block
- AAA, BBB: heap blocks
- --->: a start-pointer
- -?->: an interior-pointer

Category legend:
- DR: Directly reachable
- IR: Indirectly reachable
- DL: Directly lost
- IL: Indirectly lost
- (y)XY: it's XY if the interior-pointer is a real pointer
- (n)XY: it's XY if the interior-pointer is not a real pointer
- (_)XY: it's XY in either case

(4.2.7. 内存泄漏检测，Valgrind 用户手册)

事实证明，警告“可能丢失”涵盖了上面的案例 5-8(针对 BBB) block 。

This means that a chain of one or more pointers to the block has been found, but at least one of the pointers is an interior-pointer. This could just be a random value in memory that happens to point into a block, and so you shouldn't consider this ok unless you know you have interior-pointers.

(4.2.7. 内存泄漏检测，Valgrind 用户手册)

因此，我们以一种相当冗长的方式得出与 fbafelipe 相同的结论，即；假设您正确使用 API，或者 sqlite 泄漏了一点内存，或者它正在参与上述有效案例之一。鉴于 sqlite 项目的成熟度，可以安全地假设警告不会引起太多关注。

如果您提供有关如何使用 api(以及在什么情况下发生泄漏)的更多信息，其他人可能能够提供更多见解。

引用: Valgrind 3.6.1 source, doc/faq.html, doc/mc-manual.html